In the past, flight control systems used in many aircraft relied on direct mechanical linkages between the pilot's control devices and the aircraft flight control surfaces. In such systems, pilot-manipulated flight control devices, such as pedals, levers, and the control column, would cause connected mechanical linkages to transmit movement of the pilot control devices to the appropriate aircraft flight control surface, such as the rudders, ailerons, and elevators. Such a rudimentary system of flight control allowed for direct control of the aircraft flight control surfaces and was typically a highly reliable system. However, the mechanical flight control system required frequent, intensive inspection due to the wear and tear on the mechanical linkage, added an undesirable amount of weight to the aircraft, and required a large amount of space to properly operate.
With the advent of analog and digital circuitry, a new flight control system, commonly referred to as “fly-by-wire” or FBW, was developed whereby electronic signals generated in response to the pilot-manipulated control devices could be sent to electronic control devices to process and control movement of the flight control surfaces of the aircraft. The implementation of the fly-by-wire aircraft control system reduced the amount of weight added to the aircraft and provided a smaller footprint in the aircraft as well.
Generally, in conventional fly-by-wire aircraft control systems, electronic signals are generated by the pilot-manipulated control devices, such as the control column, and sent to a flight control computer (FCC), which also receives electronic signals from aircraft sensors providing information related to the aircraft's speed, altitude, angle of attack, and the like. The FCC processes the received electronic signals and generates a correlating instruction signal and sends the instruction signal to one or more actuator control electronics (ACE), where the instruction signal is further processed and transmitted to the appropriate actuator, which may be mechanically coupled to the respective aircraft flight control surface, e.g., the rudder, aileron, or elevator. The instruction signal provided by the FCC causes the actuator to move the aircraft control surface to correspond to the input provided by the pilot, and in some cases, the input provided by the pilot and augmented by the input from the FCC. In such cases in which the FCC augments input provided by the pilot, the fly-by-wire aircraft control system is commonly referred to as operating in “normal mode.”
Although the fly-by-wire aircraft control systems are easier to maintain than mechanical systems, the signals generated by modern control fly-by-wire systems can be very complex, and certain failures of electronic subsystems may lead to loss of operational control. In addition, the data buses and/or wires interconnecting the control electronics, actuators, and sensors may become damaged or disconnected, thereby causing interference with, or loss of, the pilot's ability to control the aircraft.
To counter such safety concerns, conventional fly-by-wire aircraft control systems provide for a plurality of microprocessors in each of the FCCs and the ACEs to protect against “random failures.” In addition, the conventional fly-by-wire aircraft control systems typically also include a plurality of dissimilar microprocessors in each of the FCCs and the ACEs to protect against “common mode failures” and may also include a plurality of dissimilar FCCs and ACEs to protect against “generic failures.” Further yet, to provide for the safe operation of the aircraft, many conventional fly-by-wire aircraft control systems include redundant FCCs and ACEs and/or bypasses to ensure that a failure in part of the fly-by-wire aircraft control system does not cause failure of the entire fly-by-wire aircraft control system. For example, in a conventional fly-by-wire aircraft control system, the failure of one or more FCCs may provide for the bypass of the FCCs, in which the instruction signal corresponding to the input provided by the pilot may be sent directly to the corresponding actuator via one or more ACEs such that the fly-by-wire aircraft control system may be referred to as operating in “direct mode” or stick-to-surface mode.
However, the hardware, e.g., redundant FCCs, ACEs, sensors, and the associated buses created there between, added to provide a high level of fault tolerance in the fly-by-wire aircraft control system, has correspondingly added to the footprint, weight, and maintenance of the fly-by-wire aircraft control system. Such additional weight is highly undesirable in view of the extra fuel required by the aircraft. Further, the larger footprint provides for less room in the aircraft to be utilized for transporting cargo and/or people. Still yet, the additional hardware requires increased maintenance hours and an increased number of replaced components, which adds undesirable cost.
What is needed, then, is a fly-by-wire aircraft control system having a smaller footprint and providing for a reduction in weight and associated maintenance costs over conventional fly-by-wire aircraft control systems currently in use.